1. Field of the Invention
The present invention relates to a data transmitting-receiving system in which digital modulation signals are synchronously detected and then demodulated by using a carrier. More particularly, the present invention relates to such a system wherein the synchronous detection can be achieved at a receiver part of the system, without utilizing a carrier which would usually be obtained through a carrier recovery operation at the receiver part.
2. Description of the Prior Art
In a prior art data transmitting-receiving system in which digital modulation signals modulated under, for example, a phase shift keying (PSK) method, are demodulated, at a receiver part thereof, through the synchronous detection operation, it is inherent to achieve, at the receiver part, a carrier recovery from the transmission signal. The synchronous detection for the transmission signal is then performed with the use of the thus recovered carrier so as to demodulate and obtain the original data.
However, there are problems with the prior art system, in that, first, usually a very complex circuit is required to achieve the carrier recovery, which makes the whole circuit construction complicated; and, second, it is difficult to achieve the carrier recovery while maintaining the phase constant over a wide frequency band, where the frequency stability of the carrier is poor, so that a phase error is increased and the quality of the signals is inevitably deteriorated when processed in the demodulation circuit part.